Hypoxic pulmonary vasoconstriction:

The great reduction in pulmonary vascular resistance during the transition from placental to normal respiration is largely brought about by the process of hypoxic pulmonary vasoconstriction. The other choices are incorrect because the PO2 of alveolar gas is much more impoant than the PO2 of mixed venous blood, CO2 uptake is irrelevant, the constriction paly dives blood flow from poorly ventilated, not well-ventilated regions of diseased lungs, and the inhalation of nitric oxide paly reverses hypoxic pulmonary vasoconstriction.